1. Field of the Invention
This invention relates to x-ray phosphors and x-ray intensifying screens. More particularly, it relates to x-ray phosphors that can enable an impure calcium tungstate which is unsuitable for x-ray purposes to be used along with europium-activated barium fluorochloride.
2. Prior Art
Calcium tungstate has been employed as the accepted standard phosphor in medical x-ray intensifying screens for over 50 years. During this time the phosphor has been greatly improved through the development of improved synthesis techniques and purer raw materials. Elimination of the transition metals in particular is most important, as relatively small amounts quench luminescence drastically, as disclosed in "Some Aspects of the Luminescence of Solids", F. A. Kroger, Elsevier, 1940, page 138. It is also believed that ionic impurities contribute to the phosphor's "lag," that is, the phosphoresence that persists after cessation of exciting radiation. There are several methods known in the art to reduce lag, such as purposeful addition of vanadium, molybdenum, and sulfates. These additives are disclosed in U.S. Pat. Nos. 1,602,593, 1,602,594 and 2,729,606. To produce the brightest commercial CaWO.sub.4, high purity is essential combined with well-controlled firing procedures and judicious use of a lag-killing agent to ensure maximum brightness. The phosphor produced by this process is expensive, reflecting the overhead costs of purification of the raw materials and the usual multiple firings given to produce maximum brightness.
It has been known for some time that certain alkaline earth halides luminesce under x-ray excitation. U.S. Pat. No. 2,303,963 discloses a procedure for preparing barium fluorochloride. That patent, however, does not mention any activator.
Great Britian Patent No. 1,161,871 and 1,254,271 also disclose processes for preparing various x-ray phosphors including europium-activated barium fluorochloride. Netherlands Patent 7,206,945 discloses a variety of alkaline earth halides activated by europium.
In view of the foregoing description of producing a commercial CaWO.sub.4 phosphor, it is clear that a phosphor of an equivalent brightness which is more economical would be an advancement in the art.
Furthermore, it is also believed to be an advancement in the art that mixtures of x-ray grade calcium tungstate and europium-activated barium fluorochloride enable screens to be made which have a brightness superior to either phosphor used individually.